The present invention is directed to a two stage planetary final drive that is operatively coupled to a hydraulic motor for driving a wheel of a work vehicle.
Motor graders are large work vehicles. Typically they are provided with a main frame that is pivotally coupled to a forwardly extending frame about a vertical pivot axis. The main frame is provided with four driven wheels. Two longitudinally separated wheels are located on each side of the main frame. The main frame houses the internal combustion engine for powering the grader. The forwardly extending frame is supported on two front wheels. The forwardly extending frame is provided with a drawbar having a grader circle. A grader blade is mounted to the grader circle by a blade frame. The position of the grader blade relative to the grader frame is controlled by hydraulic cylinders. A saddle is mounted to the forwardly extending frame above the grader circle. The saddle is provided with hydraulic cylinders for manipulating the position of the grade circle relative to the forwardly extending frame.
Some graders are all wheel drive machines. Hydraulic wheel motors drive the front wheels. The motors selectively engage the front wheels by hydraulic clutches. The front wheels may be driven in an overdrive mode, where they are driven at a faster speed than the rear wheels; at an equal speed mode where they are driven at the same speed as the rear wheels; or in an underdrive mode, where they are driven slower than the rear wheels. The assignee of the present patent application manufactures an all wheel drive motor grader of the type discussed above. The clutch/brake in the front wheel final drive is triggered by twelve circumferentially arranged axially extending pistons. The arrangement of the final drive assembly causes the front wheel fastening assembly to be different from the rear wheel fastening assembly. This forces the operator to carry two different spare wheels and tire assemblies, one for the front and one for the rear, when operating in remote areas.
One manufacturer of all wheel drive motor graders uses mechanical overrunning clutches in place of hydraulically applied clutches.
It is an object of the present invention to provide a compact final drive assembly having a wide range of speed outputs.
The final drive is contained within the circumference of a wheel that is the same size as the rear drive wheels, and uses the same fastening dimensions. In this way an operator need only carry one spare wheel and tire assembly that fits all wheels.
The final drive assembly comprises a stationary housing to which a rotatable housing is mounted. The rotatable housing in turn is coupled to the disc of the drive wheel. A bent axis two speed hydraulic motor is mounted to the stationary housing. The motor drives a first stage sun pinion which drives two stages of planetary gear trains, which in turn drive a second stage planet carrier mounted to a rotatable housing.
A second stage sun pinion is mounted coaxially to the first stage sun pinion. The second stage sun pinion freely floats radially on the first stage sun pinion. The second stage sun pinion is not driven by the first stage sun pinion. The external splines/teeth of the second stage sun pinion are coupled to and drive the internal splines of the first stage planet carrier, and drive the circumferentially arranged second stage planet gears. The second stage planet gears are freely rotatably mounted to the second stage planet carrier, which is coupled to the rotatable housing, and engage a clutch brake ring gear. The clutch/brake ring gear is operatively coupled to a plurality of clutch plates that extend inwardly and engage a clutch pack located on the stationary housing. A ring piston is used to compress the clutch pack coupling the clutch/brake ring gear to the stationary housing. The second stage planet gears coupled to the rotatable housing by the second stage planet carrier, drive the second stage planet carrier and thereby the rotatable housing and wheel.
The ring piston for applying the clutch is extended by pressurized hydraulic fluid directed through a hydraulic conduit formed in the stationary housing. The pressurized hydraulic fluid is received from the motor inlet. In the underdrive mode, the motor operates at a speed that is a preselected amount slower than the synchronous speed of the vehicle. This causes the front wheel to try to drive the motor, resulting in low motor inlet pressure, and the clutch/brake slipping. As the vehicle rear wheels lose traction and s lip to a greater degree than the preselected amount, the front wheels slow to the synchronous speed of the vehicle, motor inlet pressure rises, and the front wheel drive system provides traction without the need for any attention by the operator. Conversely, if the overdrive mode is selected, the front wheels attempt to pull the rear wheels, the motor inlet pressure is high, the clutch brake is locked, and the front wheels are continually driven by their motors. When the front wheel drive system is shut off, the motor i s stopped and the clutch/brake released thereby allowing the planetary gear system to idle. Effecting the clutch/brake function via the second stage ring gear permits the entire gear train to idle at low speed, with resultant low churning losses, when the front wheel drive system is shut off. When operating in the underdrive mode, the above mentioned clutch/brake slip occurs at low speed and low toque, with low slip power dissipation and churning losses.